1. Field of the Invention
The present invention relates to a control method employed in a combined cycle electric power generating plant, for controlling the amount of steam employed when using cooling steam to cool high temperature parts such as the combustion chamber of a gas turbine, and for controlling steam pressure in an exhaust heat recovery boiler which generates steam using exhaust heat gases.
2. Description of the Related Art
There are conventionally known systems for use in a combined cycle electric power generating plant employing gas turbines in which exhaust heat from the gas turbine is supplied to an exhaust heat recovery boiler, steam is generated inside a drum in the boiler, and the steam turbine is operated using this steam, with cooling subsequently taking place along the gas turbine's high temperature parts (combustion chamber, etc.). The method for controlling the steam line in this arrangement is a significant problem, however.
In the case where a portion of the intermediate pressure steam in an exhaust heat recovery boiler is supplied as cooling steam to the combustion chamber of a gas turbine, Japanese Patent Application, First Publication No. Hei 9-13917 discloses a method for adjusting the amount of intermediate pressure steam generated. In this method, the amount of intermediate pressure steam generated is increased by tightening a high-pressure governor valve in the steam turbine.
U.S. patent application Ser. No. 5,471,832 discloses a method for controlling the cooling steam supplied to a gas turbine in the case where one combined cycle electric power generating plant is combined with another combined cycle electric power generating plant.
Japanese Patent Application No. Hei 8-285115 (Device and Method for Controlling Gas Turbine Steam Cooling System) submitted by the current applicants discloses a method for detecting the steam temperature at the exit of the high temperature parts of the gas turbine, and controlling a control valve provided on the entrance side of the high-temperature parts, so that the aforementioned detected temperature achieves a predetermined value.
In addition, Japanese Patent Application No. Hei 8-289907 (Gas Turbine Steam System), also submitted by the present applicants, discloses a system provided with a pressure difference detecting device for the heat exchanger for the high temperature parts in the gas turbine; a protective control valve provided to the steam line which passes through the heat exchanger and goes to the steam condenser of the high and intermediate pressure steam turbines; and a temperature controlling device for receiving the signal output from the pressure difference detecting device, and sending a control signal for controlling the temperature of the heat exchanger to the protective control valve.
The conventional control methods described above have the following problems, however.
(1) In the case of a method for controlling the amount of steam supplied to the high temperature parts based on the temperature of the steam at the exit of the high temperature parts, sufficient cooling of the parts is not possible if the temperature of the steam at the exit becomes too high. Conversely, if the steam temperature becomes too low, the temperature of the steam supplied to the steam turbine falls, and the efficiency of the steam turbine decreases. Accordingly, the temperature of the steam at the exit of the high pressure parts must be controlled to an appropriate set value. However, if the value in the case where the load on the gas turbine is constant is designated as this set value, then the steam temperature value will not be appropriate if the load on the turbine changes.
(2) The amount of steam required to cool the high temperature parts increases when the load on the gas turbine rises. However, in conventional combined plants where plant efficiency is emphasized, there is little leeway for ensuring this additional quantity of steam.
(3) When the steam pressure in an exhaust heat recovery boiler becomes extremely high, a safety valve operates to protect the exhaust heat recovery boiler, releasing steam into the atmosphere. Some of this release includes the steam for cooling the high temperature parts, however, so that a shortage of cooling steam results.
The present invention was conceived in consideration of the above-described circumstances, and has as its first objective, the provision of a steam temperature control method which operates in response to changes in the load on the gas turbine. The present invention has as its second objective, the provision of a method for controlling steam pressure which is capable of accumulating steam in the exhaust heat recovery boiler when the gas turbine is operating under a constant or falling load. Finally, the present invention has as its third objective, the provision of a method for controlling steam temperature which operates in response to pressure when the steam pressure in the exhaust heat recovery boiler becomes extremely high.